One of the great mysteries of the plastics art has been, until now, an understanding of why isotactic polypropylene (PP) and polyethylene (PE) are immiscible even though they are chemically so similar. Understanding, controlling, and using the factors affecting the melt-miscibility of one polyolefin with another, for example with polypropylene, have been long-sought goals in the art. Theories abound to explain the miscibility between polymer pairs having dipolar, hydrogen-bonding, or ionic interactions. These are types of interactions which bring significant exothermic (enthalpic) contributions to the free energy of mixing. However, no theory has proven effective at explaining the general lack of miscibility of pairs of polyolefins, the polyolefins having no significant enthalpic contributions to their free energy of mixing, nor has a practical application been made in producing miscible blends of polyolefins based on our newly discovered understanding of the miscibility behavior of the polyolefins.
The problem of predicting and achieving melt-miscibility among blends of polyolefins, particularly when PP is one of the polyolefins, and achieving a concomitant improvement of certain mechanical properties in the resulting blend, had not been overcome previously. The present invention provides the composition of such blends and a process to make miscible blends of two or more polyolefins. (The inventors have reviewed several theories which have not proven applicable to the polyolefin miscibility problem and have provided some theoretical background related to this invention, in Macromolecules, 1992, 25, 5547-5550.) Some workers in the art indeed have attempted to produce useful polyolefin blends (see, for example, U.S. Pat. No. 4,226,952), but they have not recognized that some of their blends might be miscible, nor did they identify a preference for those that are now understood to be miscible in the melt. The present invention also describes how to make miscible blends in combination with other polymers and how to take advantage of the inventive process to produce blends having improved properties and performance characteristics over their less miscible blend counterparts. Application of the now-understood miscibility of certain polyolefins results in practical blends having functionalized, block or graft polymers containing a miscible polyolefin segment.